The present disclosure relates to a reproducing device that carries out signal reproduction by so-called homodyne detection and an optical path length servo control method thereof.
As an optical recording medium to/from a signal is recorded/reproduced by light irradiation, so-called optical discs such as compact disc (CD), digital versatile disc (DVD), and Blu-ray Disc (registered trademark, BD) have been spread.
Regarding the optical recording medium that should be responsible for the next generation of the optical recording media that have been spread currently, such as CD, DVD, and BD, the present assignee has proposed an optical recording medium of a so-called bulk recording type like those described in Japanese Patent Laid-Open No. 2008-135144 and Japanese Patent Laid-Open No. 2008-176902 (hereinafter, Patent Document 1 and Patent Document 2, respectively).
The bulk recording refers to a technique for increase in the recording capacity. Specifically, for example as shown in FIG. 16, an optical recording medium (bulk recording medium 100) having at least a cover layer 101 and a bulk layer (recording layer) 102 is irradiated with laser light with sequential change of the focal position, to thereby perform multilayer recording in the bulk layer 102.
Regarding the bulk recording, a recording technique referred to as a so-called micro-hologram system is disclosed in Patent Document 1.
In this micro-hologram system, a so-called hologram recording material is used as the recording material of the bulk layer 102. As the hologram recording material, e.g. a photopolymerized photopolymer is widely known.
The micro-hologram system is classified roughly into a positive micro-hologram system and a negative micro-hologram system.
The positive micro-hologram system is a technique in which two opposing beams (beams A and B) are collected to the same position to form a minute interference pattern (hologram) and this interference pattern is used as the recorded mark.
The negative micro-hologram system is based on the opposite idea of the positive micro-hologram system and is a technique in which an interference pattern formed in advance is erased by laser light irradiation and this erased part is used as the recorded mark. In this negative micro-hologram system, initialization treatment for forming an interference pattern for the bulk layer 102 in advance is performed before recording operation. Specifically, as this initialization treatment, beams of collimated light are emitted opposed to each other and an interference pattern of them is formed in the whole of the bulk layer 102.
After the interference pattern is formed in advance by the initialization treatment in this manner, information recording by forming of the erasure mark is performed. Specifically, laser light irradiation in association with recording information is performed with focus on an arbitrary layer position to thereby perform information recording based on the erasure mark.
The present assignee has proposed also a recording technique in which a void (vacancy, hole) is formed as the recorded mark like that disclosed in Patent Document 2 as a bulk recording technique different from the micro-hologram system.
The void recording system is a technique in which the bulk layer 102 composed of a recording material such as a photopolymerized photopolymer is irradiated with laser light with comparatively high power to record a vacancy (void) in the bulk layer 102. As described in Patent Document 2, the vacancy part thus formed has a refractive index different from that of other part in the bulk layer 102 and the optical reflectance is enhanced at the boundary of these parts. Therefore, this vacancy part functions as the recorded mark. This realizes information recording by forming of the vacancy mark.
In such a void recording system, the hologram is not formed and thus light irradiation from one side is sufficient for recording. That is, there is no need to collect two beams to the same position for forming the recorded mark differently from the positive micro-hologram system.
Furthermore, compared with the negative micro-hologram system, the void recording system has a merit that the initialization treatment is unnecessary.
Although an example in which irradiation with pre-cure light before recording is performed for the void recording is shown in Patent Document 2, void recording is possible also when the irradiation with pre-cure light is omitted.
In the optical disc recording medium of the bulk recording type (referred to also as bulk type simply), for which the above-described various kinds of recording techniques have been proposed, the recording layer (bulk layer) of such optical disc recoding medium of the bulk recording type does not have an explicit multilayer structure defined in the sense that plural reflection films are formed for example. That is, in the bulk layer 102, reflection film and guide groove for each recording layer, like those included in a normal multilayer disc, are not provided.
Therefore, in the case of the structure of the bulk recording medium 100 shown in FIG. 16, focus servo control and tracking servo control cannot be carried out in recording at the timing when a mark has not yet been formed.
Therefore, in practice, the bulk recoding medium 100 is provided with a reflection surface (reference surface) that has a guide groove like that shown in FIG. 17 and serves as the basis.
Specifically, the guide groove (position guide element) obtained by forming e.g. pit and groove is formed into the form of a spiral or concentric circles on the lower surface side of the cover layer 101 and a selective reflection film 103 is deposited on the guide groove. On the lower layer side of the cover layer 101 on which the selective reflection film 103 is thus deposited, the bulk layer 102 is stacked with the intermediary of a bonding material such as a UV (ultraviolet)-curable resin as an intermediate layer 104 in the diagram.
By the forming of the above-described guide groove based on pit, groove, and so forth, absolute position information (address information) such as radial position information and rotation angle information is recorded. Hereinafter, the surface in which such a guide groove is formed and the absolute position information is recorded (in this case, the surface on which the selective reflection film 103 is formed) will be referred to as “reference surface Ref.”
As shown in FIG. 18, the bulk recording medium 100 having the above-described medium structure is irradiated with servo laser light (referred to also as servo light simply) as laser light for position control separately from laser light for mark recording (or reproducing) (hereinafter, referred to also as recording/reproducing laser light or as recording/reproducing light simply).
As shown in the diagram, the recording/reproducing laser light and the servo laser light are irradiated onto the bulk recording medium 100 via a common objective lens.
At this time, if the servo laser light reaches the bulk layer 102, possibly mark recording in this bulk layer 102 is adversely affected. Therefore, in the bulk recording system of the related art, laser light in a wavelength band different from that of the recording/reproducing laser light is used as the servo laser light. In addition, as the reflection film formed on the reference surface Ref, the selective reflection film 103 having such wavelength selectivity as to reflect the servo laser light and transmit the recording/reproducing laser light is provided.
Based on the above-described premise, operation in mark recording in the bulk recording medium 100 will be described below with reference to FIG. 18.
First, when multilayer recording is performed with the bulk layer 102 in which guide groove and reflection film are not formed, which position is employed as the layer position at which a mark is recorded in the depth direction of the bulk layer 102 is decided in advance. FIG. 18 exemplifies the case in which, as the layer position at which a mark is formed in the bulk layer 102 (mark forming layer position, referred to also as information recording layer position), five information recording layer positions L from a first information recording layer position L1 to a fifth information recording layer position L5 are set. As shown in the diagram, the first information recording layer position L1 is set as the position distant from the selective reflection film 103 (reference surface Ref), on which the guide groove is formed, by a first offset of-L1 in the focus direction (depth direction). The second information recording layer position L2, the third information recording layer position L3, the fourth information recording layer position L4, and the fifth information recording layer position L5 are set as the positions distant from the reference surface Ref by a second offset of-L2, a third offset of-L3, a fourth offset of-L4, and a fifth offset of-L5, respectively.
In recording at the timing when a mark has not yet been formed, it is impossible to carry out focus servo control and tracking servo control for the respective layer positions in the bulk layer 102 based on reflected light of the recording/reproducing laser light. Therefore, focus servo control and tracking servo control of the objective lens in recording are carried out based on reflected light of the servo laser light in such a manner that the spot position of the servo laser light follows the guide groove in the reference surface Ref.
However, it is necessary that the recording/reproducing laser light reaches the bulk layer 102 formed on the lower layer side of the reference surface Ref for mark recording. Therefore, in the optical system of this case, a focus mechanism for recording/reproducing light to independently adjust the in-focus position of the recording/reproducing laser light is provided separately from the focus mechanism of the objective lens.
FIG. 19 shows the outline of the optical system for recording and reproducing of the bulk recording medium 100, including the mechanism for independently adjusting the in-focus position of the recording/reproducing laser light.
In FIG. 19, the objective lens shown in also FIG. 18 is permitted to be displaced in the radial direction of the bulk recording medium 100 (tracking direction) and the direction along which the objective lens gets closer to or remoter from the bulk recording medium 100 (focus direction) by a two-axis actuator as shown in the diagram.
In FIG. 19, a focus mechanism (expander) in the diagram serves as the mechanism for independently adjusting the in-focus position of the recording/reproducing laser light. Specifically, this focus mechanism as the expander includes a fixed lens and a movable lens that is so held by a lens driver as to be capable of being displaced in the direction parallel to the optical axis of the recording/reproducing laser light. Through driving of the movable lens by the lens driver, the collimation of the recording/reproducing laser light incident on the objective lens in the diagram changes. Thus, the in-focus position of the recording/reproducing laser light is adjusted independently of the servo laser light.
Furthermore, the wavelength band is different between the recording/reproducing laser light and the servo laser light as described above. Therefore, in association with this, the optical system of this case is so configured that the reflected light of the recording/reproducing laser light and the servo laser light from the bulk recording medium 100 is split into the respective systems (i.e. each reflected light can be independently detected) by a dichroic prism in the diagram.
In view of the forward light, the dichroic prism has a function to combine the recording/reproducing laser light and the servo laser light onto the same axis and make them incident on the objective lens. Specifically, in this case, the recording/reproducing laser light passes through the expander and is reflected by a mirror as shown in the diagram and thereafter is reflected by the selective reflection surface of the dichroic prism to be incident on the objective lens. The servo laser light is transmitted through the selective reflection surface of the dichroic prism and is incident on the objective lens.
FIG. 20 is a diagram for explaining servo control in reproducing of the bulk recording medium 100.
In reproducing of the bulk recording medium 100 in which a mark has been already recorded, there is no need to control the position of the objective lens based on the reflected light of the servo laser light differently from recording. That is, in reproducing, focus servo control and tracking servo control of the objective lens can be carried out based on the reflected light of the recording/reproducing laser light for a mark sequence formed at the information recording layer position L (in reproducing, referred to also as information recorded layer L) as the reproducing subject.
In the above-described manner, in the bulk recording system, the bulk recording medium 100 is irradiated with the recording/reproducing laser light for recording and reproducing of the mark and the servo light as light for position control via a common objective lens (with combining onto the same optical axis). Based on this configuration, in recording, focus servo control and tracking servo control of the objective lens are so carried out that the servo laser light follows the position guide element of the reference surface Ref. In addition, the in-focus position of the recording/reproducing laser light is separately adjusted by the focus mechanism for recording/reproducing light. This allows mark recording at a desired position (depth direction and tracking direction) in the bulk layer 102 although the position guide element is not formed in the bulk layer 102.
In reproducing, focus servo control and tracking servo control of the objective lens based on the reflected light of the recording/reproducing laser light are so carried out that the in-focus position of the recording/reproducing laser light follows an already-recorded mark sequence. This allows reproducing of the mark recorded in the bulk layer 102.
In the case of employing the above-described bulk recording system, the reflectance of the recorded mark is very lower than that of the recording layer in the optical disc of the related art, such as BD.
So, employing a so-called homodyne system (homodyne detection system) as the reproducing system of the bulk recording medium 100 is being studied.
As is well known, the homodyne system is a technique of performing detection of light obtained by making coherent light (DC (direct current) light) as reference light interfere with light as the detection subject (signal light) to thereby achieve signal amplification.
A technique referred to as so-called differential detection is combined with this homodyne system. Specifically, light obtained by making reference light in phase with signal light interfere with the signal light and light obtained by making reference light in anti-phase with the signal light interfere with the signal light are individually received and the difference between these light reception signals is obtained. This allows achievement of both signal amplification and noise suppression.
In this case, because the homodyne system is based on utilization of the light interference effect, the optical path length difference between the signal light and the reference light should be set shorter than at least the coherence length for realization of the homodyne system. Therefore, in the homodyne system, optical path length servo control for keeping the optical path length difference between the signal light and the reference light constant at a predetermined value is carried out.
Regarding such optical path length servo control, descriptions are made also in e.g. Japanese Patent Laid-Open No. 2008-243273 and Japanese Patent Laid-Open No. 2008-269680.